Thanks to its simplicity and cost efficiency, wireless local area network (WLAN) enjoys unique advantages in providing high-speed and low-cost wireless services in hot spots and indoor environments. Traditional WLAN medium-access-control (MAC) protocols assume that only one station can transmit at a time: simultaneous transmissions of more than one station causes the destruction of all packets involved. By exploiting recent advances in PHY-layer multiuser detection (MUD) techniques, it is possible for a receiver to receive multiple packets simultaneously. This paper argues that such multipacket reception (MPR) capability can greatly enhance the capacity of future WLANs. In addition, it provides the MAC-layer and PHY-layer designs needed to achieve the improved capacity. First, to demonstrate MUD/MPR as a powerful capacity-enhancement technique, we prove a "super-linearity" result, which states that the system throughput per unit cost increases as the MPR capability increases. Second, we show that the commonly deployed binary exponential backoff (BEB) algorithm in today's WLAN MAC may not be optimal in an MPR system, and that the optimal backoff factor increases with the MPR capability: the number of packets that can be received simultaneously. Third, based on the above insights, we design a joint MAC-PHY layer protocol for an IEEE 802.11-like WLAN that incorporates advanced PHY-layer blind detection and MUD techniques to implement MPR
The last decade has witnessed a surge of interest in wireless local area networks (WLAN), where mobile stations share a common wireless medium through contention-based medium access control (MAC). In WLANs, collision of packets occurs when more than one station transmits at the same time, causing a waste of bandwidth. The recent advances in multiuser detection (MUD) techniques [1] open up new opportunities for resolving collisions in the physical (PHY) layer. For example, in CDMA [2] [2] or multiple-antenna [3] systems, multiple packets can be received simultaneously using MUD techniques without collisions. WLAN capacity can be expected to improve greatly with such techniques. To date, most previous work on MUD has been restricted to the PHY layer. To fully utilize the multipacket reception (MPR) capability for capacity enhancement in WLAN, however, it is essential to understand the fundamental impact of MPR on the MAC-layer design. With this backdrop, this paper is a first study of the MAC-layer throughput performance and the collision resolution schemes for WLANs with MPR.
The key contributions of this paper are as follows:
To demonstrate MUD/MPR as a powerful capacity-enhancement technique at the system level, we analyze the MAC-layer throughput of WLANs with MPR capability under both finite-node and infinite-node assumptions. In contrast to previous work in [4]- [7], our model is sufficiently general to cover both carrier-sensing and non-carrier-sensing networks. We prove that in randomaccess WLANs, network throughput increases super-linearly with the MPR capability of the channel. That is, throughput divided by M increases as M increases, where M is the number of packets that can be resolved simultaneously. The super-linear throughput scaling implies that the achievable throughput per unit cost increases with MPR capability of the channel. This provides a strong incentive to deploy MPR in next-generation wireless networks.
We study the effect of MPR on the MAC-layer collision resolution scheme. When packets collide, an exponential backoff (EB) scheme is commonly used to schedule the retransmissions, in which the waiting time of the next retransmission will get multiplicatively longer for each collision incurred. In the commonly adopted binary exponential backoff (BEB) scheme (e.g., used in Ethernet [16], WiFi [17], etc.), the multiplicative (a backoff factor) is equal to 2. We show in this paper that BEB does not necessarily yield the close-to-optimal network throughput when the wireless channel can accommodate more than one simultaneous packet transmission. Instead, the optimal backoff factor heavily depends on the relative durations of idle, collision, and success slots. BEB is far from optimum for both non-carrier-sensing networks and carrier-sensing networks operated in basic access mode. The optimal backoff factor increases with the MPR capability. Meanwhile, BEB is close to optimum for carrier-sensing networks when RTS/CTS access mode is adopted.
Built on the theoretical underpinnings established above, we propose a practical protocol to fully exploit the MPR capability in IEEE 802.11-like WLANs. In contrast to [8]- [9], we consider not only the MAC layer protocol design, but also the PHY-layer signal processing to enable MPR in distributed random-access WLANs. As a result, the proposed protocol can be implemented in a fully distributed manner with marginal modification of current IEEE 802.11 MAC.
The first attempt to model a general MPR channel in random-access wireless networks was made by Ghez, Verdú, and Schwartz in [4]- [5] in 1988 an 1989, respectively. The stability properties of slotted ALOHA systems with MPR were studied under a simple infinite-user and single-buffer assumption. In 2005, Naware et al studied the stability and delay of finite-user ALOHA systems over both symmetric and asymmetric MPR channels [6]. In [7], the authors studied the stability and capacity regions of MPR networks when an optimal MAC protocol is available. The previous analyses have mainly focused on the stability of networks with MPR. Moreover, they are limited to either a specific MAC protocol, namely ALOHA [4]- [6], or an idealized optimal MAC [7]. Little work has been done to investigate the effect of MPR on the performance of practical random-access wireless networks with backoff schemes.
Our paper here is an attempt along this direction. In addition, the super-linearity result as a motivating factor for MPR in this paper is also new.
Protocols that exploit the MPR capability of networks have been studied by Zhao and Tong in [8]- [9].
In [8], a multi-queue service room (MQSR) MAC protocol was proposed for networks with heterogeneous users. The drawback of the MQSR protocol is its high computational cost due to updates of the joint distribution of all users’ states. To reduce complexity, a suboptimal dynamic queue protocol was proposed in [9]. In both protocols, access to the common wireless channel
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